Extract donor imputers from US pipeline

[MaxGhenis]

Summary

• move ColumnwiseQRFDonorImputer and RegimeAwareDonorImputer into pipelines/donor_imputers.py
• keep the names re-exported from microplex_us.pipelines.us so existing imports and monkeypatches keep working
• make the optional regime-aware backend check target the missing microimpute.models.zero_inflated submodule and skip those tests when unavailable

Verification

• UV_FROZEN=1 uv run --extra dev --extra policyengine ruff check src/microplex_us/pipelines/us.py src/microplex_us/pipelines/donor_imputers.py tests/pipelines/test_regime_aware_donor_imputer.py
• UV_FROZEN=1 uv run --extra dev --extra policyengine pytest -q tests/pipelines/test_zi_qrf_backend.py tests/pipelines/test_regime_aware_donor_imputer.py tests/pipelines/test_us.py::TestUSMicroplexPipeline::test_integrate_donor_sources_can_use_zi_qrf_backend
• npx gitnexus detect-changes --scope staged --repo /private/tmp/microplex-us-clean-slate-1